The principal structure of a telecommunication network using the W-CDMA (Wideband-Code Division Multiple Access) protocol is known from practical knowledge. The structure and function of networks of that kind is known from ITU-T Recommendation I.362.2. Networks of this kind play on important role in the so called “Third Generation” telecommunication networks.
These kinds of networks use the AAL-2 (=ATM Adaptation Layer) over ATM standards as the protocols for broadband transmission. There are usually a variety of different base stations within the W-CDMA network each of which defining a cell. A cell, especially a cell covering an area with dense traffic can be divided into a number of sectors. Each of the sectors contains its own termination point for the AAL over ATM data. To each call in a given sector of a base station a call identification (call ID, CID) is allocated and a number of calls within a given sector is mapped into a single ATM virtual connection under use of the call ID.
The multiplexing/mapping of the different calls into one single ATM virtual connection at the respective termination point is governed by a control unit. The control unit comprises a control unit timer (CU timer) which supervises the mapping of the calls into the ATM cell which has a given length (53 bits, were 48 bits are reserved for the payload). Each ATM cell is filled with the data coming from different calls within a given sector until the ATM cell is full or the allowed CU timer limit is achieved which means that the given CU time is expired. The reason for the timer is to guarantee a maximum delay in the process of filling up the ATM cells in case if there are not enough data from the calls to be transmitted. This means, if the CU timer time expires, when the ATM cell is not yet filled, the ATM cell is transmitted only partially filled which results in a reduced efficiency.
This means, that a small value of the CU timer has the advantage that only small delay in the process of filling up the ATM cells is achieved but, that the probability of generating only partially filled cells is increased.
On the other hand, if the CU timer is set to a larger value, the probability of generating only partially filled cells is reduced but this does not guarantee the required maximum delay time. In an extrapolation, if the time delay of the CU timer would be infinite, the probability of filling all cells is 100 percent, which would result in the best efficiency of the ATM network.
An other problem raising is that the amount of simultaneous calls that a single sector of a base station has to handle may not be enough to make the usage of the ATM bandwidth effective enough. Theoretical valuations have shown that the use of AAL-2 over ATM technology is only efficient if the number of CIDs in a single ATM virtual connection is at least 50.